Conventional turbogenerators have a rotor and a stator. The rotor is wound with field windings, which are disposed in slots in the body of the rotor. The stator is wound with stator coils, which are disposed in slots in the body of the stator. When the rotor is rotated by an external source of mechanical energy, such as a steam turbine or a gas turbine, and an excitation current is provided to the field windings, electrical energy is induced in the stator coils.
Stator coils are generally constructed from a plurality of individual conductors referred to as strands. The strands are stacked together to form a larger conductor (or coil) capable of carrying high voltages and currents. In many stator coils, the strands are twisted into a weaved pattern rather than simply being stacked one on top of another. This weaving technique is known as Roebelling. Roebelling helps prevent the inner strands of a stator coil, which are closest to the rotor, from carrying more current (and generating more heat) than the outer strands, which are further from the rotor. Roebelling helps ensure that each strand carries a similar amount of current and generates a similar amount of heat.
Roebel fillers are insulating materials used for filling and smoothing the irregular surfaces formed by these Roebeled strands. Roebel fillers generally comprise resin impregnated felt materials or mica splitting, which are bonded to the uneven coil surfaces by a process in which the impregnated materials are simultaneously heated and compressed, causing thermosetting resins to liquefy and fill voids of the irregular coil surfaces, and then to harden. The formed and bonded coil at this stage of manufacture is referred to as a “bakelized coil.” Following completion of the bonding stage, a mica tape ground wall is formed around the consolidated coil and vacuum-impregnated with an epoxy resin. The coil is then pressed and baked to a final cure of the insulating structure.
Because the Roebel fillers are processed and cured to the coil surfaces under atmospheric conditions, air bubbles can be entrained in the fillers, forming voids in the hardened epoxy. Voids within the insulation system of high voltage stator coils can be a source of electrical discharges either during electrical testing or during operation of the coil in an electric machine. Electrical discharges or corona activity within a coil's insulation system can be detrimental to the integrity of the insulation and lead to early failure of the coil. In air cooled coils, partial discharges within the high voltage coil can also lead to early failure of the complete stator winding.
Following the manufacture of the high voltage coils, each coil is subject to a series of electrical tests. One test that all coils must pass is the measurement of power factor tip-up. The power factor tip-up test results indicate how well consolidated the coil is, and the effect of the voids within the coil. A high voltage is applied to the coil under test, and the power factor is measured using a power factor bridge. Any internal discharges that occur in the coils cause an increase in the coils' power factor tip-up. The most likely source of partial discharges is unimpregnable, closed voids that are typically trapped bubbles in the epoxy resin associated with the Roebel filler material as mentioned above. Since the coil is bakelized at normal ambient pressure (1 atm), the hardened epoxy resin traps some air. If trapped air in the cured epoxy resin is in the electric field path upon application of the power factor test voltage, then partial discharge activity can occur and cause a high power factor tip-up. The magnitude of power factor tip-up is a function of the level of applied voltage, the size of the void, and its position in the Roebel filler material.
The most effective way to eliminate electrical discharges in the insulating structure of a high voltage stator coil, and reduce power factor tip-up, is to theoretically produce an insulating structure which is void free. Since most voids are formed within the Roebel filler material by trapped air bubbles, one solution would be to process the filler coil in a vacuum. Unfortunately, this approach is typically cost prohibitive due to the expense associated with vacuum/heat impregnation.